1. Technical Field
The present invention relates to valves, and in particular, to valves for use with turbine engines.
2. Description of the Related Art
Gas turbine engines employ a set of rotating turbine blades to compress air leading to a combustion can into which fuel is injected and ignited. Fuel is delivered through metering orifices to burners in the combustion chamber under pressure through a fuel line. Combustion of the fuel in the combustion can turns a downstream set of blades from which energy is extracted and which can also be used to drive the compressor blades.
Many gas turbine engines are operable in liquid fuel (such as diesel fuel) and gaseous fuel (such as natural gas) modes. When the engine is to be shut down or when the engine is operated in gas mode, the fuel pump supplying liquid fuel is turned off, a fuel line valve closed and the fuel line and the burner nozzles are purged with hot gases from the turbine compressors. The purging air cools the burner nozzles as well as prevents “coking” at the metering orifices, which occurs when the volatile components of the fuel are driven off by heat such that only a tarry deposit remains. Purging is necessary to prevent the burners from being damaged as well as to ensure that the system orifices and valves are clear of such deposits which could inhibit proper conduit of the fuel when the engine is returned to fuel mode.
In order to permit purging of the fuel line, a purging air line must join with the fuel line. It is important that the fueling and purging operations be isolated so that fuel does not go down the air purge line and hot gases do not travel up the fuel line to the fuel supply. Check valves are often installed in each of these lines to prevent this. However, common spring-loaded checks may be unreliable, sticking in the open position or allowing some backwash into either of the fuel or purge lines. Alternatively, the fuel cut-off valve may be operated by a pneumatic actuator in line with the check valve of the fuel line, however, this is also subject to coking and may introduce substantial pressure drops in the fuel flow. The fuel cut-off valve also introduces a potential failure point to the turbine where, if pressure is lost to the pneumatic actuator, the turbine will cease operating.
A three-way purge valve is disclosed in U.S. Pat. No. 6,050,081, assigned to the assigned of the present invention and hereby incorporated by reference as though fully set forth herein, which provides a significantly more reliable mechanism for air purging and fuel control, both preventing backwash and being more resistant to coking. As disclosed, a spool valve having an enlarged middle section was used to shuttle between positions alternatively blocking the combustion can from either the purge air line or the fuel line. The spool is biased to close off the fuel line and is urged to open the fuel line by a pilot air actuated piston. Thus, when fuel is to be closed off from the engine, the spool valve will return to its initial position thereby allowing the burner nozzles and the downstream side of the spool to be purged to reduce or eliminate coking in these areas.
Typical gas turbine combustion cans have multiple burners or combustion zones which must be separately supplied with fuel. A distributor is thus mounted to the combustion can to act as a manifold to which the several fuel lines connect before the fuel is routed to the individual burners or zones in the combustion can. This additional component and additional lines add significant cost, assembly and size to the system. And, these parts create additional areas for coking to occur, particularly given that the distributor is typically mounted directly to the combustion can which realizes extreme temperatures during combustion.
Accordingly, an improved valve is needed that has the advantages of the three way purge valve described above but with reduced assembly, size and potential for coking.